1. Field of the Invention
The present invention relates to an image processing apparatus having display and printing devices to output a binary image.
2. Related Background Art
A conventional computer basically prints only characters.
A character code is sent from a conventional computer to a printer, the character code is converted into a character bit image, and the character bit image is output using a printing head, laser scanning, and the like. In recent years, computer processing capacity has been remarkably increased, and graphics using a color image such as a natural image can be expressed.
A color image cannot be printed using simple binary colors. A natural expression of a color image cannot be achieved without expressing it as a multivalued image.
It is, however, difficult for many printers such as an ink-jet printer to perform multivalued expressions due to their structural limitations. For this reason, multivalued expressions (halftone expressions) must be achieved by binary printing.
To print a color image such as a natural image, multi-value-binary conversion is performed, and then the converted image is printed.
To display a color image, a multivalued display operation is generally performed because a display device such as a CRT can easily perform multivalued expression. It is, however, difficult for a passive matrix flat display device such as a low-cost LCD to perform multivalued expression.
Software processing cannot be standardized due to different multivalued data expression methods in display and printing, resulting in inconvenience.
In displaying a high-resolution image, multivalued expressions cause a decrease in display speed or an increase in file size because the multivalued expressions require a large memory capacity. For example, to perform a 24-bit display (using, e.g. 8-bit R, G, and B components) at a resolution of 200 dpi in an A4 size, a frame memory requires a capacity of 12 Mbytes. This capacity causes overload of the state-of-the-art computer processing capacity.
Printing and display resolutions have been increasing year by year. When a multivalued image is displayed and printed, the memory and processing capacities required are assumed to increase in arithmetic progression.
It is convenient to express a multivalued image in a binary system in printing and displaying a high-resolution image. To express a multivalued image in a binary system, a halftone expression method such as a dither method or an error diffusion method must be used. This method requires processing between pixels constituting an image (i.e., produces for each pixel a value which is based on the input values of several pixels), and a considerably large processing capacity is required to perform this pixel processing using only software (i.e., without at least some dedicated hardware).
For this reason, multi-value-binary conversion is generally performed by hardware.
Various figures and outline fonts used in vector graphics commands can be drawn at a higher speed using an external figure drawing processor than using a printing source computer or the internal processor of a printer.
Judging from the above circumstances, display and printing operations must be efficiently processed in one block, which functions as various circuits, in favor of a high-speed operation. However, the image display and printing operations are not always simultaneously performed, and different images may be displayed. Therefore, a multi-value-binary conversion circuit must independently perform display processing and printing processing.